Conventionally, in a kind of a toothed belt, for example, there is a well known compressive engagement type belt disclosed in U.S. Pat. No. 4,108,011 issued Aug. 22, 1978. As shown in FIG. 12, a belt B' is provided with a belt body 1' made of elastomer material reinforced with at least one high elastic tensile member (not shown) which extends along a belt pitch line L' and defines the belt pitch line L'. Along an inner periphery of the belt body, there are formed a plurality of synchronous belt teeth 2' at a certain pitch and each belt tooth has a width W' at tooth root smaller than the pitch. A flank part of each belt tooth 2' is formed in a circular arc with a radius R'.sub.2 equal to the width W' at tooth root, centers of the flank part 2' being on intersectioins Q and Q between the belt pitch line L' and imaginarily extended arcs of an outline of the belt tooth 2'. During engagement of the belt B' with the pulley P', as shown in FIG. 13, each belt tooth 2' meshes with a pulley groove P'.sub.1 of the pulley P' under a compressed and deformed condition. In the figure reference R.sub.1 means a radius dimension of a circular arc at a tooth root part 2'a of the belt tooth 2'.
Each tooth top part 2'b is compressed and deformed during engagment since the flank part of the belt tooth 2' of the belt B' is formed in a circular arc and a tooth height dimension of the belt tooth 2' is larger than a depth dimension of the pulley groove P'.sub.1 . Therefore, the belt tooth 2' is protected from a formation of cracks at the tooth root part 2'a and the increase of the wear at the tooth top part 2'b, and also the vibration and a noise are reduced. Namely, the since the resistivity against the shearing force at the tooth root 2'a of the belt tooth 2' increases due to the compressive force Fc applied at the belt tooth 2' during engagement, therefore, the same effect can be obtained as when using a high shearing strength member, and durability of the belt tooth 2' is improved.
In the abovedescribed conventional belt B' the tooth top part 2'b is usually flat-shaped. However, in that case, it is confirmed that there is a disadvantage due to intereference between the belt tooth 2' of the belt B' and the pulley groove P'.sub.1 during engagement. As a result of our study and the repeated experiments, it is cleared there are problems as described below and the further improvement are required to increase durability of the toothed belt and to lower the noise when the belt and the pulley are in mesh.
When the belt B' engages with the pulley P', as shown in FIG. 13, a first contact point S' at which the belt 2' first contacts with the pulley groove P'.sub.1 is on the tooth top part 2'b. However, in case the tooth top 2'b is flat-shaped, the first contact point S' is apart from the center line L.sub.2 of the pulley groove P'.sub.1. Therefore, with respect to a component of the force F applied to the tooth top part 2'b on the belt during the period ranging from the first contact between the tooth top part 2'b on the belt B' and the pulley groove P'.sub.1 to the entire engagement therebetween, the shearing component Fs applied to the tooth top part 2'b is large, and the compressive componeont Fc applied thereto is small and deviated from a center line L.sub.1 of the belt tooth 2' causing the stress concentration. For this reason, the belt tooth 2' is severly fatigued and its durability is lowered.
Also, since the first contact point S' is apart from the center line L.sub.1 of the belt tooth 2', an angle of rotation of the puley is large during the period ranging from the first contact between the tooth top part 2'b on the belt B' and the pulley groove P'.sub.1 to the entire engagement therebetween to increase the slip quantity. Therefore, the fatigue of the belt tooth 2' increases and the noise also increases.
Especially, as shown in FIG. 13, in case a bottom part of the pulley groove P'.sub.1 of the toothed pulley P' is formed in a convex circular arc whose center lies on the center line L.sub.2 of the pulley groove P'.sub.1 in an opposite side of the pulley pitch line, namely, in the same side as the pulley center, the compressive force during engagement of the belt tooth 2' with the pulley groove P'.sub.1 is deviated from the belt tooth center line L.sub.1 causing the stress concentration toward the tooth root. Therefore, the belt ltooth 2' is easily chipped. Also, when plus switch errors exist in the engagement, as shown in FIG. 14, or minus pitch errors exist therein, as shown in FIG. 15, causing the slip out of position of the exact engagement between the belt tooth 2' and the pulley groove P'.sub.1, a movement of the belt tooth 2' increases causing the large noise since the direction in which the compressive force applies largely deviates from the belt tooth center line.
The principal object of this invention is therefore to improve durability of the belt and to provide the toothed belt which can lower a noise level during engagement of the belt with the pulley by improving a shape of the belt tooth of the toothed belt, particularly, a shape of the tooth top part.
Another object of this invention is to provide a power transmission device composed of said toothed belt and a toothed puley, particularly to provide a preferable toothed pulley adapted for engaging with the toothed belt.